Daily changes in photoprotective mechanisms were studied in sun leaves of Quercus suber L., Quercus ilex L., Olea europaea L. and Eucalyptus globulus Labill. trees during the summer in Portugal. Even though stomatal closure explained most of the diurnal variation in carbon assimilation along the summer, a decline in the photochemical yield of photosystem II (F′v/F′m) also occurred, as a result of an excess of intercepted solar radiation when carbon assimilation is limited by stomatal closure due to high vapour pressure deficits and/or soil water deficits. These changes were accompanied by the conversion of violaxanthin to antheraxanthin and zeaxanthin which were correlated with thermal dissipation of excess photon energy. In spite of a common general response, differences between species were observed ‐Olea europaea, which is a slow‐growing tree, had the lowest net photosynthetic rates, the highest proportion of carotenoids in relation to chlorophyll and the highest rates of de‐epoxidation of violaxanthin. This enabled a large thermal dissipation of the excess intercepted radiation but led to rather small values of light utilisation for photochemistry (ca 20%). In contrast, in E. globulus, a fast‐growing tree, photosynthetic rates were the highest, thermal dissipation of absorbed radiation the lowest and maximal values of light utilisation for photochemistry reached ca 50%. The two Quercus species exhibited an intermediate response. A high degree of co‐ordination is apparent between stomatal behaviour, photosynthetic capacity and photoprotection mechanisms.

We compared the metabolic responses of leaves and roots of two Eucalyptus globulus Labill. clones differing in drought sensitivity to a slowly imposed water deficit. Responses measured included changes in concentrations of soluble and insoluble sugars, proline, total protein and several antioxidant enzymes. In addition to the general decrease in growth caused by water deficit, we observed a decrease in osmotic potential when drought stress became severe. In both clones, the decrease was greater in roots than in leaves, consistent with the observed increases in concentrations of soluble sugars and proline in these organs. In roots of both clones, glutathione reductase activity increased significantly in response to water deficit, suggesting that this enzyme plays a protective role in roots during drought stress by catalyzing the catabolism of reactive oxygen species. Clone CN5 has stress avoidance mechanisms that account for its lower sensitivity to drought compared with Clone ST51.

ABSTRACTto A -C i response curves. CO 2 enrichment had a small reduction effect on the development of the maximum CO 2 fixation capacity by Rubisco ( V Cmax ), and no effect over maximum electron transport capacity ( J max ), day-time respiration ( R d ) and Triose-P utilization (TPU). However, there was a statistically significant effect of N fertilization and the interaction CO 2 × × × × N over the evolution of V Cmax , J max and TPU. Relative stomatal limitation (estimated from A -C i curves) was higher (+20%) for plants grown under ambient CO 2 than for plants grown under elevated CO 2 . There was a significant effect of CO 2 and N fertilization over total biomass accumulation as well as leaf area. Plants grown at elevated CO 2 had 27% more biomass than plants grown at ambient CO 2 when given high N. However, for plants grown under low N there was no significant effect of CO 2 enrichment on biomass accumulation. Plants grown under low N also had significantly higher root : shoot ratios whereas there were no differences between CO 2 treatments. The larger biomass accumulation of Q. suber under elevated CO 2 is attributable to a higher availability of CO 2 coupled to a larger leaf area, with no significant decrease in photosynthetic capacity under CO 2 enrichment and elevated N fertilization. For low N fertilization, the effects of CO 2 enrichment over leaf area and biomass accumulation are lost, suggesting that in native ecosystems with low N availability, the effects of CO 2 enrichment may be insignificant.